There is widely known a method of gas-chromatographic analysis of mixtures of solutes involving the use of a capillary chromatographic column, through which a flow of inert carrier gas is continuously passed, and comprising the steps of periodically introducing a sample of a mixture to be analyzed into the chromatograph column and detecting separated components of said analyzed mixture at the column output by the use of a known detector (cf. U.S. Pat. No. 2,920,478).
In the foregoing method, the capillary chromatographic column is a capillary tube of a suitable inert material with an internal diameter of 0.23 to 0.53 mm, its internal surface being coated with a thin layer (0.2 to 1.0 .mu.) of a nonvolatile (low-volatile) high-molecular weight liquid which is the stationary liquid phase.
With such a method, the chromatographic column has to be replaced each time it is necessary to change selectivity of chromatographic separation in switching to the analysis of a new class of mixtures, say in the event of the change-over from the analysis of nonpolar organic compounds to the analysis of polar compounds. To enable the analysis of a wide range of substances of different polarity, the device for carrying out the foregoing method should comprise several replaceable capillary chromatograph columns filled with different stationary phases. Production of each column entails a complicated process of filling it with a stationary phase, which calls for utilization of special facilities and requires high skill on the part of the operator.
Moreover, it should be borne in mind that the cost of such a column is fairly high: a highly efficient filled capillary chromatographic column costs about a thousand dollars on the world market. Inasmuch as the known device comprises a set of several capillary chromatograph columns with different phases, its cost drastically increases.
With the foregoing method, the change-over to capillary chromatographic columns having a small internal diameter (less than 100 .mu.) and, accordingly, higher effectiveness (up to 100 thousand theoretical plates per meter) is limited by the production techniques involved. This limitation is due to the fact that, with a smaller internal diameter of the capillary column, increasing difficulties are encountered in uniformly coating the internal surface of such columns with a viscous high-molecular liquid. More specifically, the known techniques for applying the stationary liquid phase make it hard to obtain theoretically predicted highly efficient slot capillary columns by etching rectangular channels in silicon plates.
Furthermore, in the known method the capillary chromatograph columns spontaneously change their separation properties in the course of time. Stability of the separation properties of such capillary columns and their service life are dependent upon many factors, the most important of which are the operating temperature of the column, concentration of oxygen and water vapours in carrier gas, chracteristics of the material of which a particular column is made and composition of an analyzed mixture. It is common knowledge that the presence of a large amount of solvent, particularly water in a sample of a mixture to be analyzed causes rapid elution of the layer of the stationary liquid phase from the first section of the capillary column and, in effect, impairs the separation properties of the column.
In operation according to the above method at high column temperatures required to separate high-boiling organic compounds, the column acts as a source of intricate gas evolution (including residual solvent, monomers for stationary-phase synthesis and products of catalytic thermal breakdown), which increases the detector noise.
When the column is subjected to programmed heating, special techniques are required to suppress said noise in the course of the analysis procedure. Vapors of the stationary phase accumulated in the detector change its operating parameters in the course of time whereby it should be periodically cleaned or replaced. An adverse effect of said gas evolution is particularly strong in the case of a mass-spectrometric detector.
There is also known a method of gas-chromatographic analysis of mixtures with packing chromatograph columns, which comprises the steps of continuously passing a mixture of inert carrier gas and water vapors through the chromatograph column, introducing periodically a sample of a mixture to be analyzed into said column and detecting the separated components of said mixture at the column output (cf. M. S. Vigdergaus et al.: "Gasovaya khromatographiya s neidealnimi eluentami", 1980, Nauka publishers, Moscow, pp 75-99).
With such a method, the chromatograph column is a tube of a suitable inert material filled with particles of a solid carrier (Chromosorb W). The column temperature is maintained below the water-vapor dew point and water condensed on the carrier particles serves as the stationary phase in separating and analyzing polar organic compounds. The foregoing method of gas-chromatographic analysis does not ensure high efficiency in analyzing substances, a disadvantage attributable to the presence of many pores in carrier particles increasing elution of the chromatographic strips.
Moreover, with the known method the packing chromatograph column rapidly changes its separation properties with time due to the fact that water acting as an active solvent elutes the dissolved components of the solid carrier, thereby changing its surface, a factor preventing reproduction of chromatographic analysis data with time.
Other known techniques for gas-chromatographic analysis of mixtures with vapors of various solvents used as a carrier gas relate to a method of gas-solid chromatography, wherein solvent vapors are used to modify the surface of solid adsorbents. The process of modifying the surface of solid adsorbents essentially consists in that the solvent vapor block or deactivate chemical or physical heterogeneity (the so-called "active centres") on the adsorbent surface. This increases the separation efficiency and often reduces the analysis time as compared with methods involving the use of an inert carrier gas. In the aforementioned method, the temperature of the chromatograph column is maintained substantially higher than the boiling point of the solvent used (cf. Journal of Chromatographic Science, Vol. 21, Aug. 1983, Ion F. Parcher "A Review of Vapor Phase Chromatography": Gas Chromatography with Vapor Carrier Gases, pp 346-351).
As is the case with another known method involving the use of packed columns, the foregoing method of gas-solid chromatography is characterized by low separation and a longer analysis time as compared with the prior-art methods of gas-chromatographic analysis, in which highly efficient capillary columns are utilized.
Also known in the art is a method of gas-chromatographic analysis of mixtures with a chromatograph column representing a hollow capillary tube, which comprises the steps of continuously passing through said tube a carrier-gas flow containing solvent vapours, periodically introducing a sample of a mixture to be analyzed mixture into the chromatograph column maintained at a predetermined temperature and detecting at the output of the chromatograph column the components of the analyzed mixture separated in said column (cf. U.S. Pat. No. 1,122,965).
There is known a device for gas-chromatographic analysis of mixtures to accomplish the foregoing method, which comprises components successively interconnected via a pipe-line, more particularly, a carrier-gas source, a means for introducing samples of an analyzed mixture into the chromatograph column representing a hollow capillary tube placed in a thermostat, and a detector, the known device being also provided with a means for supplying solvent vapors to the carrier gas flowing to the chromatograph column (cf. U.S. Pat. No. 1,122,965).
In the afore-mentioned device for accomplishing the known method, said means for supplying solvent vapors to the carrier gas flowing to the chromatograph column comprises a solvent flow booster connected with an evaporator at the input of the capillary chromatograph column and receiving the carrier gas mixed with solvent vapors at the evaporator and supplied to the chromatograph column as a vapor gas mixture. With such a vapor-gas mixture fed to the chromatograph column representing a capillary tube operating in the condensation mode (that is, at the temperature close or equal to the solvent condensation temperature), there occurs gradual growth of the condensed liquid-phase film on the walls of the capillary tube serving as the column whereby its thickness changes. As a result, the separation properties of the column spontaneously change in operation, a disadvantage bringing about unstable analysis conditions in the course of time.
Moreover, at a certain moment the layer of solvent condensed on the inner surface of the capillary tube acting as the column may block the inner section of the tube, thus forming a solvent lock, a disadvantage sharply decreasing separation efficiency and making impossibly a gas-chromatographic analysis.
Due to the above peculiarities characterizing formation of the vapour-gas mixture supplied to the chromatograph column, the foregoing device for accomplishing the known method does not permit changing (increasing) as desired the separation capacity of said column. To this end, it is necessary to replace the chromatographic column with another, say a longer one.